For a pneumatic tire for use on icy and snowy roads (studless tire), it is known that good performance on ice may be achieved by reducing a rubber hardness at a low temperature to ensure enough ground contact area with an icy and snowy road, and/or by forming an air bubble on a contact patch to eliminate a water film on an icy and snowy road. For example, air bubbles formed in large numbers in the tread rubber may absorb water from the water film on an ice surface when the tread is in contact with and pressed on the ice surface, but may release the absorbed water by a centrifugal force when the tread is separated from the ice surface. Repetition of such a process may improve on-ice performance.
Japanese Patent No. 4046678 proposes a rubber composition for a tire tread including a thermally expandable microcapsule to form such an air bubble. The thermally expandable microcapsule may be expanded by heat during vulcanization of a pneumatic tire. Thus, many air bubbles covered by the shells of the expanded microcapsules (hollow particles) may be formed in the tread rubber of the vulcanized tire.
However, the consumer demand for studless tire performance on ice is increasing in recent years and the demand for better friction performance on ice is further increasing.